Microporous membranes are widely used as membranes for separation or permselective separation of various substances and as materials for isolation of various substances. Examples of the use of the membranes may include microfiltration membranes, fuel cell separators, capacitor separators, base materials for functional membranes to allow a novel function to develop by filling pores with a functional material, and battery separators. Above all, microporous polyolefin membranes are preferable as separators for lithium-ion batteries widely used in laptop personal computers, cell phones, and digital cameras, for example.
For instance, Patent Document 1 discloses a microporous membrane with a small surface roughness. Patent Document 2 discloses a polyester film wound around a core with a small surface roughness. Patent Document 3 discloses a technique enabling a film property to be maintained during winding while preventing misalignment during winding by improving the mechanical strength and the elastic modulus of a wound film.